The invention relates to a coordinate measuring apparatus having a sensing stylus for contacting a workpiece. Furthermore, the invention relates to a touch probe for a coordinate measuring apparatus, wherein the touch probe is provided for supporting the sensing stylus and for detecting a contact between sensing stylus and workpiece. Moreover, the invention relates to a method of contacting a workpiece by means of a coordinate measuring apparatus.
Conventional coordinate measuring apparatuses include a workpiece mount for mounting a workpiece to be measured and contacted, respectively, and a touch probe which is movable in space with respect to the workpiece mount. A sensing stylus is supported on the touch probe in a rest position with respect to the touch probe, wherein the touch probe detects deflections of the sensing stylus from its rest position. For determining coordinates of a surface of the workpiece, the touch probe is moved in space with respect to the workpiece mount until a tip of the sensing stylus comes into touching contact with the surface of the workpiece. To this end, the tip of the sensing stylus is moved towards the workpiece surface until contacting of the workpiece by the sensing tip is detected by the touch probe through a deflection of the sensing tip from its rest position. The relative position of the touch probe with respect to the workpiece mount and thus, the coordinates of the workpiece surface are then determined in a suitable coordinate system. Further coordinates of surface points of the workpiece can be determined in a similar manner.
In view of a rapid scanning of the workpiece surface approaching of the sensing tip to the workpiece surface should be as a rapid relative movement between touch probe and workpiece mount. However, such approaching also has to be carefully performed in order to prevent components of the touch probe and the sensing tip from being damaged when the movement of the touch probe relative to the workpiece cannot be stopped as quickly as required upon contact with the latter.
Therefore, the sensing tip is approached to the workpiece under the inspection of a user, who may inspect with the eye that the sensing tip approaches the workpiece and may then reduce the approach speed. Furthermore, the user also selects with the eye those surface parts and geometry elements of the workpiece which currently are to be contacted. For this purpose, in conventional coordinate measuring apparatuses, the user directly inspects with his eye the sensing tip and the workpiece.
In miniaturized workpieces and complicated workpiece geometries, the direct inspection of the sensing tip and the workpiece surface possibly may be laborious or even impossible for the user.
A coordinate measuring apparatus is known from EP 0 614 517 B1, wherein the user does not inspect with the free eye the sensing tip and the workpiece, but looks at a monitor presenting an image of the region around the sensing tip. To record the monitor image, the coordinate measuring apparatus carries next to the touch probe a camera having an objective which are aligned relative to each other such that the camera provides an image of the sensing tip and the region of the workpiece surface which is approached by the sensing tip.
It has been found that the structure of this conventional coordinate measuring apparatus cannot be adapted satisfactorily to certain applications.
Accordingly, it is an object of the present invention to provide a coordinate measuring apparatus comprising an inspection optics for inspecting the sensing tip, wherein the coordinate measuring apparatus is suitable for further kinds of applications.
Furthermore, it is an object of the invention to provide a touch probe for such coordinate measuring apparatus. A further object of the invention is to provide a touch probe of the type which allows for reliable inspection of the sensing tip or/and for greater miniaturization of the touch probe. Moreover, it is an object of the invention to provide a corresponding method of contacting a workpiece.
To this end, the invention proceeds from a touch probe comprising a touch probe chassis which forms a supporting structure for components of the touch probe and which is adapted to be fixedly attached to the coordinate measuring apparatus. On the touch probe, there is provided a support for a sensing stylus, on which support a sensing stylus is mountable for contacting the workpiece and which is deflectable from a rest position with respect to the touch probe chassis. A deflection measuring system is provided, for detecting a deflection of the support for the sensing stylus with respect to the touch probe chassis, and to allow a user to inspect the tip of the sensing stylus or/and a region around the tip of the sensing stylus tip as well as its approach to the workpiece, an inspection optics is additionally mounted on the chassis.
In one aspect, The invention is based on the finding that the side-by-side attachment of inspection optics and touch probe in the above-described conventional coordinate measuring apparatus results in restrictions regarding geometry and, in particular, makes it difficult to miniaturize the system consisting of touch probe and inspection optics. Therefore, the invention is based on the idea of integrating the inspection optics into the touch probe, so that the inspection optics xe2x80x9clooks throughxe2x80x9d the touch probe.
To this end, the inspection optics is designed to also image a region around the tip of the sensing stylus. In the object plane of the inspection optics the imaged region here has a diameter which is greater than a diameter of a touching body provided at the sensing tip for contacting the workpiece. Preferably, here the diameter of the imaged region is many times the diameter of the touching body, in particular, more than five times, ten times or, more preferably, even more than twenty times the diameter of the touching body.
In one aspect, the inspection optics is designed such that it may gain a high-contrast image of a workpiece, when the latter approaches the tip of the sensing stylus tip. If necessary, an illumination is to be also provided here to illuminate the workpiece in the region around the tip of the sensing stylus. It is also possible that the workpiece be illuminated through the inspection optics itself.
In one aspect the touch probe according to the invention may be characterized in that a transverse support, i.e., a supporting component of the touch probe extending transversely to the direction of extension of the sensing stylus, is located in a beam path of the inspection optics. This transversely extending supporting component may be located, in particular, in the line of transmission of mechanical force between the support for the sensing stylus and the coordinate measuring apparatus and, in particular, in the line of transmission of mechanical force between the support for the sensing stylus and the chassis and may be part of the support for the sensing stylus or/and the chassis or/and a further component provided between chassis and support for the sensing stylus.
That the transversely extending supporting component is disposed in the beam path of the inspection optics, here means that lines of the geometric beam path between a focal plane of the inspection optics in which, for example, the sensing stylus tip may be disposed, and an entrance lens of the inspection optics, intersect this transversely extending component. Hence, this transverse support is in principle able to at least partly cover the image plane of the inspection optics. To nevertheless ensure the desired possibility of inspection through the inspection optics, the transverse support is transparent to light at least portions at portions thereof.
Hence, the tip of the sensing stylus on the one hand, and a region around the tip, on the other hand, are intended to be inspected by means of the inspection optics. To this end, the inspection optics is adapted to a length of the sensing stylus and thus, for example, to a distance between the transverse support and the tip of the sensing stylus, such that the tip of the sensing stylus is located near an object plane of the inspection optics. In particular, this is the case when an amount of a ratio of a first distance D1 between the transverse support and the object plane over a second distance D2 between the transverse support and the tip of the sensing stylus is within a range of from 0.5 to 2.0, preferably, 0.75 to 1.4 and, more preferably, 0.85 to 1.2.
This is preferably achieved by the transverse support itself being made of a material which is transparent to radiation. For this purpose, it is particularly preferred, when the transverse support itself is made of glass.
As an alternative here, it is preferably also possible to provide the transverse support with one or a plurality of apertures such that, on the one hand, its supporting function is maintained and, on the other hand, sufficient possibilities are created for radiation to pass the transverse support between the object plane and an entrance lens of the inspection optics. To this end, the transverse support may be formed by a plurality of strips of material between which gaps are provided for the passage of light. The transverse support may be formed as a net or grid, it may be formed as a perforated surface or its components providing for the supporting function may be as narrow as to maintain a sufficiently great surface for the passage of light.
Advantageously, an optical main axis of the inspection optics and a longitudinal axis of the sensing stylus substantially coincide. The viewing direction of the user on the workpiece consequently corresponds to the direction of extension of the sensing stylus.
Preferably, the part of the support for the sensing stylus on which the foot of the sensing stylus is directly mounted, is located on or near the main axis of the inspection optics. When seen in this main axis, a sufficient surface for apertures and transparent bodies, respectively, is then provided around the foot of the sensing stylus to allow the view on the sensing tip.
According to a preferred embodiment of the invention, the transverse support is elastically deformable and formed as a component to elastically couple the base bodies of the chassis and the support for the sensing stylus. The transverse support may then be provided to define the rest position of the support for the sensing stylus with respect to the touch probe chassis and to also provide a restoring force against deflections from this rest position.
With regard to a miniaturization of the arrangement and to well-defined restoring forces, the transverse support and components of the touch probe chassis and the support for the sensing stylus are integrally made from one block of material.
Preferably, the block of material is a monocrystal which is made of silicon for example. Advantageously, etching techniques are employed for producing the desired shape of the components.
In one aspect the deflection measuring system is preferably provided such that it detects elastic deformations of the transverse support and/or of regions of the touch probe chassis, or the support for the sensing stylus. To this end, preferably resistance strain gauges are provided, which are provided in regions on the transverse support or/and regions of the touch probe chassis or/and the support for the sensing stylus. It is also possible that the resistance strain gauges be integrated into these components.
Furthermore, the invention provides a coordinate measuring apparatus comprising the above-described touch probe, and the invention further provides a method of contacting a workpiece, in which, through the inspection optics integrated into the touch probe, a user is allowed to inspect the sensing stylus tip and the workpiece to be contacted, respectively.